Method and arrangement for connection re-establishment in a telecommunication system

ABSTRACT

The invention relates to a method in a user equipment and a user equipment ( 700 ) for controlling connection re-establishment between said user equipment and a network. The method comprises the steps of —receiving ( 501 ) a configuration message from the network defining at least one condition; —upon detection of connectivity problems ( 503 ) towards the first cell, evaluating said at least one condition ( 504 ); —if said at least one condition is fulfilled for a neighboring cell (B), applying at least one special criterion for initiating connection re-establishment to said neighboring cell ( 505 ), said at least one special criterion being different from at least one criterion that is applied if said at least one condition is not fulfilled. The invention furthermore relates to a method and an arrangement in a network node ( 800 ) for controlling connection re-establishment between the network and a user equipment ( 700 ).

PRIORITY

This application is a division, under 35 U.S.C. § 120, of U.S. patentapplication Ser. No. 16/414,318 filed May 16, 2019, which is acontinuation of U.S. patent application Ser. No. 14/381,492 filed Aug.27, 2014, which is a U.S. National Stage Filing under 35 U.S.C. § 371 ofInternational Patent Application Serial No. PCT/SE2012/050288, filedMar. 15, 2012, and entitled “Method and Arrangement for ConnectionRe-Establishment in a Telecommunication System” all of which are herebyincorporated by reference in their entirety.

FIELD OF THE INVENTION

The present invention relates to controlling initiation of connectionre-establishment upon detection of connectivity problems.

Communication devices such as User Equipments (UE) are also known ase.g. mobile terminals, wireless terminals and/or mobile stations. Userequipments are enabled to communicate wirelessly in a cellularcommunications network or wireless communication system, sometimes alsoreferred to as a cellular radio system or cellular networks. Thecommunication may be performed e.g. between two user equipments, betweena user equipment and a regular telephone and/or between a user equipmentand a server via a Radio Access Network (RAN) and possibly one or morecore networks, comprised within the cellular communications network.

User equipments may further be referred to as mobile telephones,cellular telephones, laptops, or surf plates with wireless capability,just to mention some further examples. The user equipments in thepresent context may be, for example, portable, pocket-storable,hand-held, computer-comprised, or vehicle-mounted mobile devices,enabled to communicate voice and/or data, via the RAN, with anotherentity, such as another user equipment or a server. The concept of userequipment also comprises devices with communication capability ofmachine-type character such as sensors, measurement devices etc that notnecessarily is in any interaction with a user.

The cellular communications network covers a geographical area which isdivided into cell areas, wherein each cell area being served by a basestation, e.g. a Radio Base Station (RBS), which sometimes may bereferred to as e.g. “base station”, “eNodeB”, “NodeB”, “B node”, or BTS(Base Transceiver Station), depending on the technology and terminologyused. The base stations may be of different to classes such as e.g.macro eNodeB, home eNodeB or pico base station, based on transmissionpower and thereby also cell size. A cell is the geographical area whereradio coverage is provided by the base station at a base station site.

LTE Mobility

Mobility management is a challenging task in cellular communicationssystems and a well functioning mobility performance is crucial to thequality experienced by the end user. The Radio Resource Controlprotocol, RRC, see 3GPP TS 36.331, is the main signaling protocol forconfiguring, re-configuring and general connection handling in the LTEradio access network (E-UTRAN). RRC controls many functions such asconnection setup, mobility, measurements, radio link failure andconnection recovery.

A user equipment, UE, in LTE can be in two RRC states: RRC_CONNECTED andRRC_IDLE. In RRC_CONNECTED state, mobility is network-controlled basedon e.g. measurements provided by the user equipment. That is, thenetwork decides when and to which cell a user equipment should be handedover, based on e.g. measurement reports provided by the user equipment.The network, i.e. the LTE radio base station, called eNB in E-UTRANconfigures various measurement events, thresholds etc based on which theuser equipment then sends reports to the network, such that the networkcan make a decision to hand over the user equipment to a stronger cellas the user equipment moves away from the present cell.

FIG. 1 illustrates a simplified signaling scheme for the LTE handover,HO, procedure. It should be noted that the HO command is in factprepared in the Target eNB, i.e. the eNodeB that the user equipment willbe handed over to, but the message is transmitted via the Source eNB.That is, from the user equipment's perspective the message comes fromthe Source eNB.

In RRC_IDLE, mobility is handled by UE-based cell-selection, where anomadic user equipment selects the “best” cell to camp on, based e.g. onvarious specified criteria and parameters that are broadcasted in thecells. For example, various cells or frequency layers could beprioritized over other, such that the user equipment tries to camp on aparticular cell as long as the measured quality of a beacon or pilot inthat cell is a threshold better than some other beacon or pilot receivedfrom other cells.

The present disclosure is primarily focusing on problems associated withnetwork-controlled mobility as described above, i.e. for an LTE userequipment in RRC_CONNECTED state. The problems associated with failinghandovers are therefore described in further detail below.

In a regular situation, and when a RRC_CONNECTED user equipment ismoving out from the coverage of a first cell, also called source cell,it should be handed over to a neighboring cell, also called target cellor second cell before loosing the connection to the first cell. That is,it is desirable that the connection is maintained with no or minimaldisruption throughout the handover, such that the end-user is unaware ofthe ongoing handover. In order to succeed with this, it is necessarythat

-   -   the measurement report that indicates the need for mobility is        transmitted by the user equipment and received by the Source        eNB, and    -   the Source eNB has sufficient time to prepare the handover to        the target cell (by, among other things, requesting a handover        from the Target eNB controlling the target cell), and    -   the user equipment receives the handover command message from        the network, as prepared by the Target eNB in control of the        target cell and sent via the source cell to the user equipment,        see FIG. 1.

In addition, and in order for the handover to be successful, the userequipment must finally succeed in establishing a connection to thetarget cell, which in LTE requires a successful random access request inthe target cell, and a subsequent transmission of a HO complete messagefrom the user equipment to the Target eNB. It should be noted thatspecifications may differ somewhat in the naming of messages.

Thus, it is clear that in order for the handover to succeed, it isnecessary that the sequence of events leading to a successful handoveris started sufficiently early, so that the radio link to the first cellover which this signaling takes place does not deteriorate too muchbefore completion of the signaling. If such deterioration happens beforethe handover signaling is completed in the source cell (i.e. firstcell), then the handover is likely to fail. Such handover failures areclearly not desirable. The current RRC specification therefore providesvarious triggers, timers, and thresholds in order to adequatelyconfigure measurements, such that the need for handovers can be detectedreliably, and sufficiently early.

In FIG. 1, the exemplified measurement report is triggered by ameasurement event called A3 event, which in short means that a neighborcell is found to be an offset better than the current serving cell. Thismeans that a measurement report is sent to the network when a criterionor criteria associated with the event is satisfied. There exists manydifferent measurement event types, and it should be noted that there aremultiple events that could be configured to trigger a report.

A network node in control of a cell, such as an eNodeB in LTEterminology maintains a neighbor cell relation list. Whenever areference is made to a neighbor cell in this disclosure, it should beunderstood as a reference to a cell typically comprised in the neighborcell relation list of a network node. A neighbor cell is thus a cellthat is often a candidate for a handover. In some cases, the networknode maintains a neighbor cell list related to each cell that it iscontrolling. From the perspective of the user equipment, a neighbor cellis a cell in the proximity or overlapping with the cell to which theuser equipment is currently connected to.

Radio Link Failure and RRC Connection Re-Establishment

It may occur that a user equipment looses coverage to the cell that theuser equipment is currently connected to. This could occur in asituation when a user equipment enters a fading dip, or that a handoverwas needed as described above, but the handover failed for one oranother reason. This is particularly true if the “handover region” isvery short, as will be further described below.

The quality of the radio link is typically monitored in the userequipment e.g. on the physical layer, as described in 3GPP TS 36.300, TS36.331 and TS 36.133, and summarized below.

Upon detection that the physical layer experiences problems according tocriteria defined in TS 36.133, the physical layer sends an indication tothe RRC protocol of the detected problems called out-of-sync indication.After a configurable number, N310, of such consecutive indications, atimer, T310, is started. If the link quality is not improved (recovered)while T310 is running, i.e. there are no N311 consecutive “in-sync”indications from the physical layer, a radio link failure, RLF, isdeclared in the user equipment, see FIG. 2.

The currently relevant timers and counters described above are listed inFIG. 4 for reference. The user equipment may read the timer values andcounter constants from system information broadcasted in the cell.Alternatively, it is possible to configure the user equipment withUE-specific values of the timers and counter constants using dedicatedsignaling, i.e. where specific values and constants are given tospecific user equipments with messages directed only to one or morespecific user equipment.

The function of the timers and counters used for monitoring radio linkfailure in LTE is presented in the tables in FIG. 3

If T310 expires, the user equipment initiates a connectionre-establishment to recover the ongoing RRC connection. This procedureincludes cell selection by the user equipment. That is, theRRC_CONNECTED user equipment shall try to autonomously find a bettercell to connect to, since the connection to the previous cell failedaccording to the described measurements. It could occur that the userequipment returns to the first cell anyway, but the same procedure isalso then executed. Once a suitable cell is selected as furtherdescribed e.g. in 3GPP TS 36.304, the user equipment requests tore-establish the connection in the selected cell. It is important tonote the difference in mobility behaviour as an RLF results in userequipment based cell selection, in contrast to the normally appliednetwork-controlled mobility.

If the re-establishment is successful, which depends on, among otherthings, if the selected cell and the eNB controlling that cell wasprepared to maintain the connection to the user equipment, which impliesthat is was prepared to accept the re-establishment request, then theconnection between the user equipment and the eNB can resume. In LTE, are-establishment procedure includes a random-access request in theselected cell, followed by higher layer signalling where the userequipment sends a message with content based on which the user equipmentcan be identified and authenticated. This is needed so that the networkcan trust that it knows exactly which user equipment is attempting toperform the re-establishment.

A failure of a re-establishment means that the user equipment goes toRRC_IDLE and the connection is released. To continue communication, anew RRC connection has then to be requested and established. A failurecould occur e.g. if the eNB that receives the re-establishment requestis not able to identify the user equipment that requests there-establishment. Such a condition may occur if the receiving eNB hasnot been informed or prepared for a possible re-establishment from thisuser equipment.

The reason for introducing the timers T31x and counters N31x describedabove is to add some freedom and hysteresis for configuring the criteriafor when a radio link should be considered as failed and need to bere-established. This is desirable, since it would affect the end-userperformance negatively if a connection is abandoned prematurely if itturned out that the loss of link quality was temporary and the userequipment succeeded in recovering the connection without any furtheractions or procedures, e.g. before T310 expires, or before the counterreaches value N310.

The re-establishment procedure will be described in the following: Thenetwork node, such as eNB, controlling a target cell receives a recoveryrequest message from the user equipment, such as an RRC connectionre-establishment request. In response to this message, the target eNBmay respond with an RRC connection re-establishment message sent to theuser equipment, by which the target eNB accepts the re-establishmentrequest. The message may include various configuration parameters, suchthat the connection can be adapted and continued in the new cell. Othermessage names may of course apply, such as any reference to cellre-selection or handover.

Upon reception of re-establishment message, the user equipment may nowprocess the content of that message, and resume the RRC connectionaccording to the content and commands provided therein. Typically, theuser equipment would further send a confirm message to the eNB of thetarget cell, where the confirm-message indicates that the communicationbetween the user equipment and the target eNB can now resume. Forexample, the RRC connection re-establishment request, there-establishment message, and subsequent confirm-message would typicallyinclude fields for supporting secure identification of the userequipment and fields for supporting contention resolution, i.e. suchthat the user equipment and its connection can be unambiguously andsecurely identified.

The recent and rapid uptake of Mobile Broadband has lead to a need forincreasing the capacity of cellular networks. One solution to achievesuch a capacity increase is to use denser networks consisting of severallayers of cells with different sizes: Macro cells ensure large coveragewith cells encompassing large areas, while micro-, pico- and evenfemto-cells are deployed in hot-spot areas where there is a large demandfor capacity. Those cells typically provide connectivity in a muchsmaller area, but by adding additional cells and radio base-stationscontrolling those cells, capacity is increased as the new cells off-loadthe macros. Such networks are referred to as Heterogeneous Networks orHetNets. FIG. 9 shows a User equipment moving from the coverage of apico-cell A into the coverage of macro cell B.

The different “layers” of cells can be deployed on the same carrier,i.e. in a reuse-1 fashion, the small cells could be deployed on adifferent carrier, and the different cells on the various layers couldeven be deployed using different technologies, e.g. 3H/HSPA on themacro- and micro-layer, and LTE on the pico-layer as one non-exclusiveexample.

There is currently a large interest for investigating the potential ofsuch HetNets. However, it has also been found that HetNets may result inan increased rate of handover failures, as briefly discussed above. Onereason is that the handover region in HetNets may be very short, meaningthat the handover might fail since the user equipment lost coverage tothe source cell before the handover to a target cell could be completed.For example, when a user equipment leaves a pico-cell, it may happenthat the coverage border of the pico is so sharp, that the userequipment fails to receive any handover command towards a macro beforeloosing coverage to the pico, see FIG. 4.

Similar problems could occur when a user equipment connected to a macrocell suddenly enters a pico cell on the same carrier. It could nowhappen that the control channels of the pico interferes with the signalsthat the user equipment needs to receive from the macro in order tocomplete the handover, and the handover thus fails.

In failed handovers of the kind exemplified above, the user equipmentwill eventually try to re-establish the RRC connection. But this canonly occur after the procedures prior to the recovery procedure havebeen completed, as described above: Thus, the user equipment willobserve “out-of-sync” on Layer 1, L1, towards the source cell, thoseevents will be counted on Layer 3, L3, level, i.e RRC, as describedabove until N310 such consecutive events have occurred, and then T310will be started. Only when T310 has expired, the user equipment caninitiate re-establishment procedure by searching for a better cell toconnect to, in order to recover the RRC connection.

It is clear that this counting (up to N310) and the waiting of T310 toexpire will result in an undesired interruption of the connectivity thatis likely to be observable for the end-user.

One could therefore argue that the network should configure the relevantcounters and timers (with N310 and T310 as non-exclusive examples) withsmall values in order to speed up the recovery. However, this couldresult in premature loss of the connection in case the radio problemswere not due to an imminent handover, but only due to a sudden fadingdip.

SUMMARY

As a consequence of the problem described in the foregoing, there is adifficulty in finding suitable criteria to trigger connectionre-establishment, such as values of relevant counters and timers, thatwould be suitable for both the handover problems described here, and themore general case of a fading channel that experiences time-varyingfluctuations in path-loss and interference.

It is therefore an objective of embodiments of the present invention toprovide a solution for reducing the number of handover failures leadingto connection release perceived by the end user.

More specifically, a first aspect of an embodiment of the inventionrelates to a method in a user equipment, UE, for controlling connectionre-establishment between said user equipment and a network, said userequipment being served in a first cell comprised in said network. Themethod comprises the steps of

-   -   receiving a configuration message from the network defining at        least one condition;    -   upon detection of connectivity problems towards the first cell,        evaluating said at least one condition;    -   if said at least one condition is fulfilled for a neighboring        cell, applying at least one special criterion for initiating        connection re-establishment to said neighboring cell, said at        least one special criterion being different from at least one        criterion that is applied if said at least one condition is not        fulfilled.

A second aspect of an embodiment of the invention relates to a method ina network node to be comprised in a network for controlling connectionre-establishment between the network and a user equipment, said networkserving the user equipment in a first cell comprised in the network. Themethod comprises the steps of

-   -   sending a configuration message to one or more user equipment        defining at least one condition;    -   configuring said one or more user equipment to evaluate said at        least one condition upon detection of connectivity problems        towards the first cell;    -   configuring said one or more user equipment to apply at least        one special criterion for initiating connection re-establishment        to a neighboring cell if said at least one condition is        fulfilled for said neighboring cell, said special criterion        being different from at least one criterion that is applied if        said at least one condition is not fulfilled.

A third aspect of an embodiment of the invention relates to a userequipment adapted for controlling connection re-establishment betweensaid user equipment and a network, said user equipment being served in afirst cell comprised in said network. The user equipment comprises

-   -   a receiver configured to receive a configuration message from        the network defining at least one condition;    -   a processor comprising circuitry configured to evaluate if said        at least one condition is fulfilled upon detection of        connectivity problems towards the first cell, and circuitry        configured to apply said at least one special criterion for        initiating connection re-establishment to a neighboring cell if        said at least one condition is fulfilled for said neighboring        cell, said at least one special criterion being different from        at least one criterion that is applied if said at least one        condition is not fulfilled.

A fourth aspect of an embodiment of the invention relates to anarrangement in a network node, to be comprised in a network, adapted forcontrolling connection re-establishment between the network and a userequipment, said network serving the user equipment in a first cellcomprised in the network. The arrangement comprises

-   -   a transmitter configured to send a configuration message to one        or more user equipment defining at least one condition;    -   processing circuitry arranged to configure said one or more user        equipment to evaluate said at least one condition upon detection        of connectivity problems towards the first cell, and to apply at        least one special criterion for initiating connection        re-establishment to said neighboring cell if said at least one        condition is fulfilled for a neighboring cell, said at lest one        special criterion being different from at least one criterion        that is applied if said at least one condition is not fulfilled.

Thus, embodiments are directed to reducing the interruption time inchallenging radio environments, where handover failures are likely tohappen. A fast re-establishment of the connection will reduce anyinterruption time experienced by an end user. Particular embodimentswill also ensure that there is a higher likelihood of a successfulconnection recovery.

Other objects, advantages and novel features of the invention willbecome apparent from the following detailed description of the inventionwhen considered in conjunction with the accompanying drawings andclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of theinvention will be apparent from the following detailed description ofpreferred embodiments as illustrated in the drawings.

FIG. 1 shows a simplified signaling diagram illustrating the LTEhandover procedure;

FIG. 2 shows how Radio Link Failure, RLF is declared in LTE;

FIG. 3 shows how RLF is monitored in LTE:

FIG. 4 illustrates the concept of handover regions of pico/macro vsmacro/macro cell change;

FIG. 5 shows a flowchart of a method embodiments according toembodiments performed in a user equipment;

FIG. 6 shows a flowchart of a method embodiments performed in a networknode;

FIG. 7 shows schematically a user equipment according to embodiments;

FIG. 8 shows schematically an arrangement in a network node according toembodiments; and

FIG. 9 shows a user equipment moving from the coverage of a pico-cellinto the coverage of a macro cell.

DETAILED DESCRIPTION

In the following description, for purposes of explanation and notlimitation, specific details are set forth such as particulararchitectures, interfaces, techniques, etc. in order to provide athorough understanding of the invention. However, it will be apparent tothose skilled in the art that the invention may be practiced in otherembodiments that depart from these specific details. In other instances,detailed descriptions of well-known devices, circuits, and methods areomitted so as not to obscure the description of the invention withunnecessary details.

Reference throughout the specification to “one embodiment” or “anembodiment” means that a particular feature, structure, orcharacteristic described in connection with an embodiment is included inat least one embodiment of the present invention. Thus, the appearanceof the phrases “in one embodiment” or “in an embodiment” in variousplaces throughout the specification are not necessarily all referring tothe same embodiment. Further, the particular features, structures orcharacteristics may be combined in any suitable manner in one or moreembodiments.

The present invention is described within the context of E-UTRAN, alsoreferred to as LTE. It should be understood that the problems andsolutions described herein are equally applicable to wireless accessnetworks and user equipments, user equipment's implementing other accesstechnologies and standards. LTE is used as an example technology wherethe invention is suitable, and using LTE in the description therefore isparticularly useful for understanding the problem and solutions solvingthe problem.

Various alternative embodiments for implementing the invention will bedescribed in the following using non-exclusive examples.

Consider a user equipment in connected mode implementing networkcontrolled mobility and connected to a first cell, e.g. a pico-cell.With the notation “connected to a cell”, it should be noted that thisimplies that the user equipment is configured to send and/or receivedata in at least this cell. Sometimes, this cell is also called “servingcell”, or “primary cell”, PCell. The cell is typically controlled by onenetwork node such as a radio base station, which means that “connectedto a cell” is in this respect equivalent to the user equipment being“connected to a first radio base station”, that is, the radio basestation that is in control of the aforementioned cell. Sometimes, thisradio base station is called Serving eNB or serving radio base station.

In the example, the pico-cell is “surrounded” by a macro-cell thatprovides coverage in a much larger area, see FIG. 9. When the userequipment is moving out from the coverage of the small pico cell it isquickly loosing coverage of the pico cell that it is currently connectedto, see FIG. 4. In such situation, there would be a rather highlikelihood that the user equipment will loose connectivity to thepico-cell before a handover is completed, which means that the userequipment will experience a radio-link failure resulting in an undesiredinterruption in connectivity, as previously described.

The same problem could also occur if a user equipment moves from a macrocell to another cell, e.g. a pico-cell, where there is a very shorthandover region. The problem would be further exaggerated if the userequipment moves with a high speed.

Embodiments of the invention provide a possibility to bypass theordinary, or “normal”, evaluation of radio link failure for performingconnection re-establishment in a particular neighboring cell provided atleast one certain condition is fulfilled for said cell.

FIG. 5 schematically illustrates method steps performed by a userequipment, UE, according to embodiments of the invention, said userequipment being served in a first cell, which in this example is thepico cell. In step 501, the user equipment receives a configurationmessage from the network defining at least one condition to be evaluatedby the user equipment upon detection of connectivity problems towardsthe first cell. Said configuration message may be received on abroadcast channel or via dedicated signaling.

Said condition may be related to determining if the user equipment iswithin the coverage of any neighboring cell. The evaluation of whetherthe user equipment is within the coverage of a neighboring cell may bebased on whether a received signal from a neighboring cell is strongerthan a particular value e.g. a threshold value. Various qualityparameters for defining the “strength” of such a received signal can beimplemented, for example received signal power measurements andpathloss-related and interference related measurement criteria. Thisincludes for example Reference symbol received power (RSRP) andReference symbol received quality (RSRQ).

The network may configure the user equipment with a threshold orthresholds that defines whether the user equipment should consideritself within the coverage region of an indicated cell. The network mayalso configure the user equipment with a quality parameter to be used bythe user equipment when evaluating whether the user equipment is withinthe coverage region of an indicated cell. If for example the RSRQ of anindicated cell in the example outlined above is greater than x dB, wherex represents a configured threshold value, then the user equipmentconsider itself within the coverage region of that cell.

If the user equipment identifies that said at least one condition isfulfilled for multiple neighboring cells, e.g. that it is withincoverage of multiple cells, there may be additional criteria forprioritizing the selection among those. For example, the user equipmentmay select the best one, where “best” is defined in terms of themeasured criterion above, e.g. such that the cell with the strongestsignal or best signal quality is selected. Alternatively, there could beadditional priority criteria defined, such as a relative priority amongcells comprised in a neighboring cell list, or offsets defining e.g.that cell A, or cells on frequency layer F_A should be selected if thecell or cells are x dB better in terms of the measured criterion whencompared to another cell B or cells on another frequency layer F_B.

As the user equipment moves out from the coverage region of the firstcell, which is the pico-cell in this example, it will detect physicallayer problems or other problems related to the connectivity towards thefirst cell, i.e. the pico-cell, see step 503. The detection of physicallayer problems may comprise receiving an out-of-sync indication from thephysical layer or reading Reference Signal Received Power, RSRP, orReference Signal Received Quality, RSRQ, below a certain absolute leveltowards the first cell. The network may configure the user equipmentwith a threshold or thresholds for RSRP or RSRQ that defines whether theuser equipment should consider the connection to the source cell to beproblematic. According to another alternative, detection of connectivityproblems towards the first cell comprises the step of starting a timerwhen a measurement event related to a handover is triggered, and if nohandover has been executed before the timer expires, connectivityproblems are detected.

In step 504, the user equipment evaluates whether said at least onecondition is fulfilled. Said condition may as already mentioned berelated to determining whether the user equipment is within the coverageof a neighboring cell. The evaluation is done by checking if certain oneor more conditions related to a neighboring cell are fulfilled as willbe further described below. In such case, the user equipment willimplement a fast connection re-establishment to such cell by applyingthe special criteria or criterion for initiating connectionre-establishment in said cell, see step 505.

In a particular embodiment a condition is fulfilled when a receivedsignal from said neighboring cell is stronger than a threshold value.

According to an alternative embodiment, a condition is fulfilled if ameasurement event, e.g. A3, is triggered for said neighboring cell.Alternatively, a condition is fulfilled for a neighboring cell if ameasurement report is transmitted to the network indicating that ameasurement event is triggered for a neighboring cell. According to yetanother alternative, a condition is fulfilled for a neighboring cell ifan acknowledgement of a successfully transmitted measurement reportindicating that a measurement event is triggered for a neighboring cellis received from the network.

A benefit of embodiments where the condition is tied to a measurementevent that is already defined for the user equipment is that no newmeasurement of a target cell needs to be defined.

A benefit of the two latter embodiments, i.e. where a condition isfulfilled when a report is transmitted and an acknowledgement of thesame is received respectively, is that they allow the network to beaware of the user equipments intentions to move to the neighboring cellwhich in this case is the target cell, and can initiate neededpreparations, such as target cell preparation. It should be noted thatthe A3 event is merely an example and that other measurement events maybe applied in the context of different embodiments.

If said at least one condition is fulfilled for a neighboring cell, thenat least one special criterion for initiating connectionre-establishment to said neighboring cell is applied, according to step505. Said special criterion is different from at least one criterion forinitiating connection re-establishment that would be applied if said atleast one condition had not been fulfilled.

If said at least one condition is not fulfilled for a neighboring cell,the user equipment shall apply the normal criterion or criteria forinitiating connection re-establishment, see step 506, and possibly do anormal re-establishment procedure based on e.g. regular cell selectionprocedures.

In an optional step 507, the user equipment receives, from the network,an indication of at least one cell for which said at least one specialcriterion should apply, provided that the at least one condition isfulfilled. Said indication may in some embodiments be comprised in theconfiguration message received in step 501.

The indication of at least one cell provided to the user equipment instep 501 or 507 may be in different forms. According to a specificembodiment, said indication comprises an indication of all neighboringcells of the first cell. In this case, there is no need to specificallyindicating, or pinpointing, particular cells, but there is merely anindication sent from the network to one or more user equipment statingthat the user equipment shall apply the special criterion or criteriaallowing the fast re-establishment solution whenever said at least onespecific condition for fast re-establishment procedure is fulfilled.

According to another specific embodiment, said indication comprises anindication of a particular frequency layer such that all neighboringcells on said frequency layer are indicated. This means that in case theat least one condition is fulfilled, the user equipment should apply thefast connection re-establishment solution towards cells on a particularfrequency layer provided that said at least one condition is fulfilled,but not on another frequency layer. Also in such cases, there may be noneed to indicate particular cells on the layer, since all neighbor cellson the particular frequency layer are indicated with a singleindication.

According to yet another specific embodiment, said indication comprisesa list of indicated cells, towards which the user equipment shallperform a fast re-establishment of the connection in case the at leastone condition is fulfilled. In case the indication is provided in theform of a list of cells, the cells in the list would be identified by acell identification, typically by their physical cell identity, PCI, butother indexes or identification methods for identifying the cells in thelist may also apply. Such list may also comprise an indication of whichfrequency layer a cell indicated by said identification is to be foundon.

The indication provided in step 507 may be provided to the userequipment on a broadcast channel, BCCH in LTE terminology, or it may beprovided by dedicated signaling, where a message is sent to a particularuser equipment or group of user equipments, but not to all userequipments in that cell. Broadcast signaling typically implies that themessage or messages are sent to some or all user equipments within thecoverage of a cell. The messages are then often repeated periodically,so that user equipments entering the cell can receive the message ormessages at the next opportunity. Dedicated signaling from the networktypically implies that the message is directed to one particularreceiver or group of receivers, by specifically addressing this userequipment or group of user equipments in the transmission of themessage. Thus, the message may be sent only once followed by anacknowledgement from the receiving user equipment.

The application of criterion or criteria to be fulfilled for initiatingconnection re-establishment may in some embodiments include the use ofcounters and or timers to evaluate when the user equipment shallinitiate connection re-establishment in an indicated cell. The countersand/or timers may be the same as those used in “normal” recoveryprocedures. For example, the network configures the user equipment witha counter value N310 equal to 10 and a timer expiration value T310 equalto 500 milliseconds. A “normal” connection re-establishment is theninitiated after 10 consecutive “out-of-synch” indications followed bythe starting and expiry of a timer T310, i.e. followed by a 0.5 secondwaiting time. Only then, the user equipment selects a cell towards whichthe user equipment performs a re-establishment request.

According to a particular embodiment, re-establishment is initiated to aneighboring cell immediately when said at least one condition isfulfilled for that cell. This means that the re-establishment isinitiated as soon as said at least one condition is fulfilled, i.e. noadditional criteria for initiating connection re-establishment isapplied. Such embodiment will be explained using the followingnon-limiting example:

A user equipment is configured by the network to initiate connectionre-establishment towards a certain neighboring cell upon detection ofconnectivity problem towards its serving cell in case an A3 event hasbeen triggered for said neighboring cell. Let us assume that theconnectivity problem is detected by reception of an out-of-synchindication from the physical layer. According to this particularembodiment, the user equipment will then stop the counter N310 countingout-of synch indications from the physical layer or timer T310controlling radio link failure and immediately initiate connectionre-establishment towards the neighboring cell for which the A3 event hasbeen triggered, i.e. towards the target cell.

In an alternative example embodiment, said at least one specialcriterion include expiration of a timer at a certain expiration value,said expiration value being different from an expiration value of saidtimer that is applied for “normal” connection re-establishment, i.e. ifsaid at least one condition had not been fulfilled, and/or reaching athreshold value of a counter, said threshold being different from athreshold value for said counter that is applied if said at least onecondition had not been fulfilled, i.e. for normal connectionre-establishment. Thus, the user equipment may implement and usedifferent values of timers and/or counters to evaluate when the userequipment shall initiate a connection re-establishment when said atleast one condition has been fulfilled.

Thus, according to embodiments of the present invention, the userequipment may perform an RRC connection re-establishment to aneighboring cell prior to finalizing the counting and expiry of thetimer, e.g. T310, according to a “normal” connection re-establishmentprocedure, provided that the at least one condition is fulfilled.

For example, the counter threshold value may as a special criterion beset to 3, such that the special criterion implies that the userequipment may initiate a re-establishment towards a neighboring cell forwhich said at least one condition is fulfilled after 3 “out-of-synch”indications. Alternatively, or in addition, the timer expiration valuemay as another special criterion be set to 100 milliseconds, such thatthe user equipment may initiate the re-establishment towards aneighboring cell for which said at least one condition is fulfilledafter 100 milliseconds from starting T310.

In another alternative embodiment, said at least one special criterioninvolves the use of at least one different counter and/or timer inaddition to at least one counter and/or timer used for “normal”initiation of connection re-establishment, said different counter and/ortimer govern when the user equipment is allowed to initiatere-establishment of the connection to a cell for which the condition isfulfilled. For example, initiation of re-establishment in a cell forwhich the condition is fulfilled could be associated with a countervalue “3” of a counter N410, and a timer-value “100 ms” of a timer“T410”. The naming of the timers and counters should be seen as examplesonly and do not imply any limitations to the applicability ofembodiments of the invention. The same counter “N410” and timer “T410”could be used for governing the recovery to all neighboring cells. Therecould also be separate values for the counter and timer of eachneighboring cell or group of neighboring cells.

Numerous variations and combinations are possible. Values of countersand timers for evaluating when the user equipment should attempt tore-establish the connection to a neighboring cell, both for the caseswhen the at least one condition is fulfilled or not fulfilled, could insome embodiments be configurable by the network, see optional steps 502in FIGS. 5 and 603 in FIG. 6.

Embodiments of the present invention also includes the network aspectsfor configuring the user equipment with at least one condition toevaluate in order to determine if a user equipment shall implement afast connection re-establishment procedure towards a particularneighboring cell. On the network side, the rationale of providing suchat least one condition to be evaluated upon detection of connectivityproblems towards the first, i.e. serving cell to a user equipment orgroup of user equipments is that handover is particularly challenging insome deployments. Thus, it often happens that one has to resort toconnection recovery as initiated by the user equipment in particularregions. Therefore, there is an incentive to speed up this recovery atplaces where such problems are likely.

FIG. 6 schematically illustrates method steps performed by a networknode according to embodiments of the invention. In step 602, the networknode sends a configuration message to one or more user equipments beingserved in a first cell that is controlled by said network node, themessage defining at least one condition. Examples of such condition havebeen described in the foregoing in relation to the description of themethod performed by the user equipment. In step 605, one or more userequipment is configured by the network node to evaluate said at leastone condition upon detection of connectivity problems towards the firstcell. In step 606, said one or more user equipment is configured by thenetwork node to apply at least one special criterion for initiatingconnection re-establishment to a neighboring cell if said at least onecondition is fulfilled for said neighboring cell, said special criterionbeing different from at least one criterion that would be applied ifsaid at least one condition had not been fulfilled.

In an optional step 607, the network node sends an indication of atleast one cell for which said at least one special criterion forinitiating connection re-establishment apply if said at least onecondition is fulfilled. The indication of at least one cell can be indifferent forms as described in the foregoing in relation to thedescription of the method performed by the user equipment.

In an optional step 600, the network node receives information from oneor more neighboring network nodes about previous re-establishmentrequests in order to establish which cells that should be indicated tothe one or more user equipment in step 607. Thus, the network usesstatistics from e.g. previous handover failures in order to establishwhich neighbor cells that should be indicated. For example, if there isinformation in an eNB that handovers from its cell A to cell B oftenfails, the network may add cell B as an indicated cell towards whichuser equipments should implement fast connection re-establishment byapplication of said at least one special criterion. The information ofe.g. failed handovers may for example be received from neighboringnetwork nodes over an X2, S1 or O&M interface.

A network node may thus be arranged to collect information aboutconnectivity problems such as handovers that have failed, in order toprovide such information to other eNBs in control of neighboring cells.For example, a network node B in control of cell B finds that userequipments often arrives in this cell after a failed handover from cellA, see FIG. 9. The network node B in control of cell B may receive thisinformation in a RLF-report message, see 3GPP TS 36.331 and theinformation element may be failedPCellId. The network node B collectsthis information, and sends it directly, or via some aggregating node,e.g. an O&M node, to the network node A in control of cell A, wherebythat network node can add cell B as an indicated cell towards which userequipments shall implement fast recovery.

In optional step 601, the network node establishes which cells thatshould be indicated to the user equipment that is sent in step 607. Thiscan be done by utilizing information or statistics of previousconnectivity problems, either received from neighboring network nodes asdescribed in the foregoing or available in itself.

In step 604, which is an optional step relating to a particularembodiment, the network prepares neighboring cells that has beenindicated to the one or more user equipments in step 607 by sending amessage to at least one second network node that is in control of suchindicated cell, e.g. a radio base station or eNodeB in LTE terminology,such that the second network node is prepared to receive connectionre-establishment requests from particular user equipments currentlyconnected to the first cell. The preparation may include sending ahandover request from the first network node to the second network node.If both cells are controlled by the same network node, no message needsto be sent, but the network node may itself be prepared for a connectionre-establishment from UE1 in an indicated cell.

Alternatively, if no preparation message is sent, a second network nodein control of an indicated cell will not receive any information relatedto any user equipment that might request connection re-establishment insaid indicated cell. Therefore, said second network node is not inpossession of any user equipment context related to such user equipment.The second network node in control of an indicated cell may then fetchthe context of such user equipment from the network node in control ofthe first cell. Such fetching implies that the second network node, upondetection of a re-establishment request from a user equipment, requestsinformation related to the user equipment from the first network node.The fetching is successful if the second network node receives necessaryinformation related to the user equipment to accept the re-establishmentrequest. When the fetching is successful, the second network node mayaccept the re-establishment from the user equipment.

A user equipment 700, UE, according to embodiments of the invention isschematically illustrated in FIG. 7. The user equipment comprises areceiver 702 and a transmitter 703. Said receiver is configured toreceive a configuration message from the network defining at least onecondition. Examples of such condition have been described in theforegoing in relation to the description of the method performed by theuser equipment. Said message can be received either on a broadcastchannel or via dedicated signaling.

The user equipment 700 furthermore comprises a processor 701 comprisingprocessing circuitry configured to evaluate if said at least onecondition is fulfilled upon detection of connectivity problems towardsthe first cell, and circuitry configured to apply said at least onespecial criteria for initiating connection re-establishment to saidneighboring cell if said at least one condition is fulfilled for aneighboring cell, said at least one special criterion being differentfrom at least one criterion that would be applied if said at least onecondition had not been fulfilled.

In some embodiments, the user equipment 700 furthermore comprises atimer 704 and/or a counter 706. In one specific embodiment, said atleast one special criterion include expiration of the timer 704 at acertain expiration value, said expiration value being different from anexpiration value of said timer 704 that is applied if said at least onecondition had not been fulfilled and/or reaching a threshold value ofthe counter 706, said threshold being different from a threshold valuefor the counter 706 that is applied if said at least one condition hadnot been fulfilled. In one particular embodiment, the timer 704 may bethe timer T310, and the counter 706 may be the counter N310.

In yet further embodiments, the user equipment 700 comprises another,different counter 707 and/or timer 705 for initiating connectionre-establishment to a neighboring cell in addition to a counter 706and/or timer 704 used for initiating re-establishment to a cell if saidat least one condition had not been fulfilled.

An arrangement in a network node 800 is schematically illustrated inFIG. 8. The network node is adapted to be comprised in a network and isfurthermore adapted for controlling connection re-establishment betweenthe network and a user equipment, UE, said network serving the userequipment in a first cell comprised in the network.

The arrangement comprises a receiver 802 and a transmitter 803. Thetransmitter 803 is configured to send a configuration message to one ormore user equipments defining at least one condition. Examples of suchcondition have been described in relation to the description of themethod embodiments performed by the user equipment. Said message can betransmitted either on a broadcast channel or via dedicated signaling.

The arrangement in the network node 800 furthermore comprises aprocessor (801) comprising processing circuitry arranged to configuresaid one or more user equipment to evaluate said at least one conditionupon detection of connectivity problems towards the first cell. Theprocessing circuitry is furthermore arranged to configure the one ormore user equipment to apply at least one special criterion forinitiating connection re-establishment to said neighboring cell if saidat least one condition is fulfilled for a neighboring cell, said specialcriterion being different from at least one criterion that would beapplied if said at least one condition had not been fulfilled.

In some embodiments, the transmitter 803 is configured to send anindication of at least one cell for which said at least one specialcriterion for initiating connection re-establishment apply if said atleast one condition is fulfilled. The indication of at least one cellcan be in different forms as described in the foregoing in relation tothe description of the method performed by the user equipment.

In some embodiments, the receiver 802 is configured to receiveinformation from one or more neighboring network nodes about previousre-establishment requests in order to establish which at least one cellthat should be indicated to the one or more user equipment.

In some embodiments, the processor (801) comprises circuitry configuredto utilize information of previous connectivity problems to establishwhich cells that should be indicated to the one or more user equipment.Said processing circuitry may also be configured to use information orstatistics received by receiver (802) from neighboring network nodesabout re-establishment requests, to establish which cells that should beindicated to the one or more user equipment.

In some embodiments, the transmitter (803) may be configured to send apreparation message to at least one neighboring network node controllingat least one cell indicated to said one or more user equipments, forpreparing such network node to receive a connection re-establishmentrequest from said one or more user equipments.

The method steps performed by the user equipment (700) and thearrangement in the network node (800) are performed by functionalelements of the processing circuitry in their respective processors 701and 801. In some embodiments these functions are carried out byappropriately programmed microprocessors or microcortrollers, alone orin conjunction with other digital hardware, which may include digitalsignal processors (DSPs), special-purpose digital logic, and the like.Either or both of the microprocessors and digital hardware may beconfigured to execute program code stored in memory. Again, because thevarious details and engineering tradeoffs associated with the design ofbaseband processing circuitry for mobile devices and wireless basestations are well known and are unnecessary to a full understanding ofthe invention, additional details are not shown here

Program code stored in the memory circuit may comprise one or severaltypes of memory such as read-only memory (ROM), random-access memory,cache memory, flash memory devices, optical storage devices, etc., andincludes program instructions for executing one or moretelecommunications and/or data communications protocols, as well asinstructions for carrying out one or more of the techniques describedherein, in several embodiments. Of course, it will be appreciated thatnot all of the steps of these techniques are necessarily performed in asingle microprocessor or even in a single module.

The present invention may, of course, be carried out in other ways thanthose specifically set forth herein without departing from essentialcharacteristics of the invention. The present embodiments are to beconsidered in all respects as illustrative and not restrictive.

The invention claimed is:
 1. A method in a user equipment, UE, forcontrolling connection re-establishment between said user equipment anda network, said user equipment being served in a first cell comprised insaid network, the method comprising: receiving a configuration messagefrom the network, the configuration message defining one or moreconditions to be fulfilled in order to apply a fast re-establishmentprocedure, wherein the one or more conditions comprise one or more ofthe following conditions: a condition that is defined to be fulfilledwhen, based on a received signal from a neighboring cell, the userequipment is within the coverage of said neighboring cell; a conditionthat is defined to be fulfilled when the received signal from saidneighboring cell is stronger than a threshold value; and a conditionthat is defined to be fulfilled when, based on the received signal fromsaid neighboring cell, a measurement event or measurement report istriggered for said neighboring cell; upon detection of connectivityproblems towards the first cell, evaluating said one or more conditions;and applying the fast re-establishment procedure or a normalre-establishment procedure with respect to said neighboring celldepending on whether the one or more conditions are fulfilled for saidneighboring cell, wherein the fast re-establishment procedure initiatesconnection re-establishment to said neighboring cell faster if said oneor more conditions are fulfilled than the normal re-establishmentprocedure that said user equipment would otherwise apply if said one ormore conditions were not fulfilled.
 2. The method according to claim 1,the method further comprising: receiving, from the network, anindication of one or more cells for which said fast re-establishmentprocedure applies if said one or more conditions are fulfilled.
 3. Themethod according to claim 1, wherein said fast re-establishmentprocedure initiates connection re-establishment to said neighboring cellat a certain expiration value of a timer, said expiration value beingdifferent from an expiration value of said timer that is applied forsaid normal re-establishment procedure.
 4. A method in a network node tobe comprised in a network for controlling connection re-establishmentbetween the network and a user equipment, said network serving the userequipment in a first cell comprised in the network, the methodcomprising: sending a configuration message to the user equipment, theconfiguration message defining one or more conditions to be fulfilled inorder for the user equipment to apply a fast re-establishment procedure,wherein the one or more conditions comprise one or more of the followingconditions: a condition that is defined to be fulfilled when, based on areceived signal from a neighboring cell, the user equipment is withinthe coverage of said neighboring cell; a condition that is defined to befulfilled when the received signal from said neighboring cell isstronger than a threshold value; and a condition that is defined to befulfilled when, based on the received signal from said neighboring cell,a measurement event or measurement report is triggered for saidneighboring cell; configuring the user equipment to evaluate said one ormore conditions upon detection of connectivity problems towards thefirst cell; and configuring the user equipment to apply the fastre-establishment procedure or a normal re-establishment procedure withrespect to said neighboring cell depending on if said one or moreconditions are fulfilled for said neighboring cell, wherein the fastre-establishment procedure initiates connection re-establishment to saidneighboring cell faster if said one or more conditions are fulfilledthan the normal re-establishment procedure that the user equipment wouldotherwise apply if said one or more conditions were not fulfilled. 5.The method according to claim 4, the method further comprising: sendingthe user equipment an indication of one or more cells for which saidfast re-establishment procedure applies if said one or more conditionsare fulfilled.
 6. The method according to claim 5, wherein the networknode utilizes information of previous connectivity problems to establishwhich cells should be indicated to the user equipment.
 7. The methodaccording to claim 5, wherein the network node receives information fromneighboring network nodes about previous re-establishment requests inorder to establish which cells should be indicated to the userequipment.
 8. The method according to claim 5, wherein the network nodesends a preparation message to at least one network node controlling atleast one cell being indicated to the user equipment, the preparationmessage for preparing such network node to receive a connectionre-establishment request from the user equipment.
 9. A user equipmentadapted for controlling connection re-establishment between said userequipment and a when said user equipment is being served in a first cellcomprised in said network, said user equipment comprising: a receiver,the receiver configured to: receive a configuration message from thenetwork, the configuration message defining one or more conditions to befulfilled in order to apply a fast re-establishment procedure, whereinthe one or more conditions comprise one or more of the followingconditions: a condition that is defined to be fulfilled when, based on areceived signal from a neighboring cell, the user equipment is withinthe coverage of said neighboring cell; a condition that is defined to befulfilled when the received signal from said neighboring cell isstronger than a threshold value; and a condition that is defined to befulfilled when, based on the received signal from said neighboring cell,a measurement event or measurement report is triggered for saidneighboring cell; and processing circuitry, the processing circuitryconfigured to: upon detection of connectivity problems towards the firstcell, evaluate said one or more conditions; and apply the fastre-establishment procedure or a normal re-establishment procedure withrespect to said neighboring cell depending on whether the one or moreconditions are fulfilled for said neighboring cell, wherein the fastre-establishment procedure initiates connection re-establishment to saidneighboring cell faster if said one or more conditions are fulfilledthan the normal re-establishment procedure that said user equipmentwould otherwise apply if said one or more conditions were not fulfilled.10. The user equipment according to claim 9, wherein said receiver isfurther configured to receive, from the network, an indication of one ormore cells for which said fast re-establishment procedure applies ifsaid one or more conditions are fulfilled.
 11. The user equipmentaccording to claim 9, wherein said fast re-establishment procedureinitiates connection re-establishment to said neighboring cell at acertain expiration value of a timer, said expiration value beingdifferent from an expiration value of said timer that is applied forsaid normal re-establishment procedure.
 12. An arrangement in a networknode, the arrangement comprising: a transmitter, the transmitterconfigured to: send a configuration message to the user equipment, theconfiguration message defining one or more conditions to be fulfilled inorder for the user equipment to apply a fast re-establishment procedure,wherein the one or more conditions comprise one or more of the followingconditions: a condition that is defined to be fulfilled when, based on areceived signal from a neighboring cell, the user equipment is withinthe coverage of said neighboring cell; a condition that is defined to befulfilled when the received signal from said neighboring cell isstronger than a threshold value; and a condition that is defined to befulfilled when, based on the received signal from said neighboring cell,a measurement event or measurement report is triggered for saidneighboring cell; and processing circuitry, the processing circuitryconfigured to: configure the user equipment to evaluate said one or moreconditions upon detection of connectivity problems towards a first cell,and configure the user equipment to apply the fast re-establishmentprocedure or a normal re-establishment procedure with respect to saidneighboring cell depending on if said one or more conditions arefulfilled for said neighboring cell, wherein the fast re-establishmentprocedure initiates connection re-establishment to said neighboring cellfaster if said one or more conditions are fulfilled than the normalre-establishment procedure that the user equipment would otherwise applyif said one or more conditions were not fulfilled.
 13. The arrangementaccording to claim 12, the transmitter further configured to transmit,to the user equipment, an indication of one or more cells for which saidfast re-establishment procedure applies if said one or more conditionsare fulfilled.
 14. The arrangement according to claim 13, wherein theprocessing circuitry is further configured to utilize information ofprevious connectivity problems to establish which cells should beindicated to the user equipment.
 15. The arrangement according to claim13, wherein: the arrangement further comprises a receiver configured toreceive information from neighboring network nodes aboutre-establishment requests; and the processing circuitry is furtherconfigured to use the information to establish which cells should beindicated to the user equipment.
 16. The arrangement according to claim13, wherein said transmitter is configured to send a preparation messageto at least one network node controlling at least one cell beingindicated to the user equipment, the preparation message for preparingsuch network node to receive a connection re-establishment request fromthe user equipment.